In recent years, the greenhouse effect caused by an increase in the CO2 (carbon dioxide) concentration has been pointed out as a factor of the phenomenon of global warming, and there has been an urgent need for international countermeasures to protect the environment in a global scale. Major sources for CO2 generation are places where industrial activities occur, and there is an increasing tendency to reduce the emission of CO2. Therefore, with regard to the facilities such as thermal power generation plants which use fossil fuels, a method of bringing the exhaust gas generated when fossil fuels (coal, petroleum, natural gas, and the like) are combusted, into contact with an aqueous alkanolamine solution or the like, and thereby removing and recovering CO2 in the combustion exhaust gas, and a method of storing the recovered CO2 have been put into practice all over the world. Furthermore, it has been suggested to remove acidic gases such as H2S (hydrogen sulfide) in addition to CO2, using an aqueous alkanolamine solution.
Examples of technologies for suppressing an increase in the concentration of acidic gases including CO2, include the development of energy saving products, technologies for separating and recovering acidic gases that have been discharged, technologies for utilizing or separately storing acidic gases as resources, and the switchover to alternative energies such as natural energies and atomic energy, which do not emit any acidic gases. As the acidic gas separation technologies that have been hitherto studied, an absorption method, an adsorption method, a membrane separation method, a cryogenic method and the like are available. Among them, the absorption method is suitable for treating large amounts of gases, and thus, application thereof to manufacturing plants or power generation plants has been under examination.
In general, alkanolamines represented by monoethanolamine (MEA) have been developed since the 1930's, as substances to be used for the absorption method, and those substances are currently still in use (Patent Literature 1). This method is economically efficient, and an increase in size of the removal devices can be easily achieved.
Examples of existing alkanolamines that are widely used include monoethanolamine, 2-amino-2-methylpropanolamine, methylaminoethanol, ethylaminoethanol, propylaminoethanol, diethanolamine, bis(2-hydroxy-1-methylethyl)amine, methyldiethanolamine, dimethylethanolamine, diethylethanolamine, triethanolamine, and dimethylamino-1-methylethanol. Particularly, since methylethanolamine as a primary amine and diethylethanolamine as a secondary amine have fast reaction rates, these compounds have been widely used all over the world. However, there are problems with these compounds in terms of corrosiveness, requirement of high regeneration energy, degradation, and the like. Furthermore, methyldiethanolamine has low corrosiveness and requires low regeneration energy, but has a defect of a low absorption rate. Under such circumstances, there is a demand for the development of a new absorbent liquid.
In regard to the recent tendency of the development of absorbent liquids, among those amine-based absorbent liquids, research is being actively conducted on alkanolamines having structural steric hindrance in particular (Patent Literature 2). It is because alkanolamines having steric hindrance are advantageous in that the selectivity of acidic gases is very high, and less energy is required for regeneration. The reaction rates of sterically hindered amines are dependent on the degree of impediment of reaction determined by the steric structure of the compounds, and the reaction rates are lower than those of primary and secondary amines such as methylethanolamine and diethanolamine, but are higher than those of tertiary amines. For example, as alkanolamines used in absorbent liquids, 2-amino-2-methylpropanol, 2-piperidineethanol, and the like are known.
On the other hand, there are known methods of using cyclic amines that are different in structure from alkanolamines, as absorbents (Patent Literature 1 and Patent Literature 3).